Along with the burst of penetration of the mobile communication devices, the request for miniaturization at lower cost has been increasing. For this reason, it is desired to apply an integrated circuit the integration density of which has been improved by decreasing voltage control type oscillators (VCO) and a number of filters. As one of conventional examples of transmitters, there has been dual band transceiver IC high-frequency technique (See non-patent literature 1, K. Takikawa et. al. “RF Circuits Technique of Dual-band Transceiver IC for GSM and DCS1800 applications”, IEEE 25th European Solid-State Circuits Conference, 1999, pp. 278-281) for GSM, DCS1800 proposed by Takikawa et al.
As an important item in the design of the transmitter circuit, there is cited reduction in noise leakage to a receive frequency band. In the case of, for example, an European portable telephone GSM, noise in a reception band spaced apart by only 20 MHz from an upper limit of a transmission band with respect to maximum output power 33 dBm must be suppressed to be equal to or less than −79 dBm/100 kHz (−129 dBm/Hz). If a band pass filter or the like is applied to the output unit of a power amplifier, the above-described specification will be able to be achieved, but the efficiency will be reduced under the influence of lost filter. For this reason, as structure using no filter, there has been applied an offset PLL system.
Structure of a transmitter to which a conventional offset PLL system has been applied is shown in FIG. 17 (See, for example, non-patent literature 2, T. Yamawaki et al, “A 2.7-V GSM RF Transceiver IC”, IEEE J. Solid-State Circuits, December 1997, Vol. 32, No. 12, pp. 2089-2096).
The transmitter is composed of an intermediate frequency (IF) signal generating unit 1703 and a PLL (Phase Locked Loop) unit 1702.
First, an operation of the IF unit will be described. To an I-signal input terminal 113, a Q-signal input terminal 114, there is inputted I, Q signal each having a band of 200 kHz. This input signal is mixed with an IF local oscillation signal 1706, 1707 having a phase difference of 90° in a mixer 117, 118.
In this case, the IF local oscillation signal is obtained by shifting the phase of output from an oscillator 1108 by 90° by a phase shifter 110. By adding the output of respective mixers, it is converted to a GMSK (Gaussian Minimum Shift Keying) modulation signal of IF frequency (270 MHz). The GMSK modulation signal is a modulation signal which has been adopted in a GSM (Global Systems for Mobile Communications) system, and has signal information only in the phase at constant amplitude. In order to provide a phase comparator 1102 in the latter stage with sufficient amplitude, the IF signal is amplified in an amplifier 1101. After a higher harmonic which occurs in the mixer 117, 118 and an amplifier 1101 is eliminated by a low-pass filter 1705, the IF signal is inputted into the phase comparator 1102 of the PLL portion 1702.
The PLL portion is characterized by including a mixer 1103, and mixes frequency 915 MHz of an output signal from a voltage control oscillator (VCO) 1104 which operates at RF frequency with a local oscillation frequency (fL0) 1185 MHz by the mixer 1103 to thereby convert into an IF frequency (270 MHz) for outputting an error with the IF signal through the phase comparator 1102. The frequency of the error signal outputted lowers to the same base band signal band as the IQ input signal.
High-frequency noise of the error signal is suppressed by a low-pass filter (LPF) 1106. A cut-off frequency of a closed loop 1701 of the filter is about 1.6 MHz against a signal band of 200 kHz, and noise of 20 MHz is suppressed to a large extent. For this reason, noise in a band spaced apart by 20 MHz from an output signal from the VCO1104 is suppressed to a large extent. Therefore, even if output from the VCO is directly connected to a power amplifier (PA) 924, it becomes possible to suppress noise to a reception band to −79 dBm/100 kHz (−129 dBm/Hz) or less without newly connecting a filter to a RF signal, and it is possible to transmit a signal of a transmission frequency (fTX) 915 MHz through an antenna 1704.
Although the offset PLL has been broadly applied without necessitating any exterior filter for high frequency as described above, it has been difficult to apply to a modulation system in which information is included even in such a change in amplitude as HQPSK (Hybrid Quadrature Phase Shift Keying) which has been adopted in W-CDMA (Wideband CDMA (Code Division Multiple Access).
Also, it has been difficult to realize a change in signal level of 70 dB or higher which is requested in the W-CDMA.